A text editor should be used whenever you’re updating files for your site. A program that saves text with formatting (like Word or textEdit) isn’t a good option, as improper characters can break your site.
For both Windows and Mac (and Linux, if you’re in to that sort of thing!) I work in Sublime. You can download and evaluate it for free, but committing to it is $70. If you want a completely free alternative, take a look at Atom.